1. Field of the Invention
This invention relates to production of high purity, low attenuation optical fibers, and is particularly concerned with novel high silica optical fibers and methods of fabricating such fibers employing a radial vapor deposition technique.
It is well known that light can be caused to propagate along a waveguiding structure comprising a transparent core which has a higher refractive index than a surrounding transparent cladding. Very small diameter optical fibers based on this principle are increasingly being used to transmit extremely wideband modulated signals over long distances. Optical fibers produced for these purposes must have low signal attenuation (e.g. less than 1 dB/Km), which inherently requires not only that the materials be of high light transmissivity but also that defects in the form of seeds (a term widely used in the glass industry to designate bubbles) and other discontinuities be minimized. The fibers should also introduce low dispersion in the transmitted light, and should allow only preselected modes of light to propagate along the fiber.
Producing satisfactory optical waveguiding fibers has been one of the more difficult problems in the development of modern optical communication systems. At this time the primary applications of optical fibers are in telephony, but the technology is being extended to many other data transmission applications such as CATV, computers and other industrial and military applications. Single mode optical systems are particularly suited for high capacity long distance communication systems because of the extremely wide bandwidths and uniform transmission characteristics they provide. A very thorough discussion of the operating theories of single mode fiber is contained in the publication "Understanding Monomode Optical Fibers" by A. W. Snyder, IEEE Proceedings, Vol. 69, No. 1, p. 6 (1981). Other excellent sources of information concerning single mode fiber and transmission systems are "Components and Systems for Longwavelength Monomode Fibre Transmission," by Garrett and Todd, Optical and Quantum Electronics, 14, p. 95-143 (1982) and "Low Loss Single Mode Fiber Development and Splicing Research in Japan", by H. Murata and N. Inagaki, IEEE Journal of Quantum Electronics, Vol. 17, No. 6, p. 835 (1981).
Low loss single mode fibers for high capacity long distance transmission systems are presently virtually all made of high silica vapor deposited glass. The primary reason for using high silica glass is its optical transmission characteristics, primarily its low scattering in the near infra-red region of the optical spectra where suitable light sources are available. The advantages of vapor deposition technology reside in the purity of the glass that can be produced and the control of dimensions and refractive indices of the core and cladding that can be achieved. Most transmission systems using optical fibers are based on wavelengths between 600 and 1600 nanometers (nm), with 1300 nm currently being widely employed.
2. Description of the Prior Art
The prior art discloses optical fibers or waveguides using high silica materials and methods of making high purity high silica glass by chemical vapor deposition followed by sintering to provide optical fiber preforms which can be drawn into optical fibers of suitably small dimensions. The desirability of making waveguiding optical fibers with high silica materials in which the core contains silica of higher refractive index than a pure fused silica outer cladding was first identified by E. Ronald Schineller. This work is described in his report "Summary of the development of optical waveguides and components", NASA CR-860, published (1967) by Clearinghouse for Federal Scientific & Technical Information, Springfield, Va. 22151. Subsequently, others adapted chemical vapor deposition techniques to this particular application, and a number of variations are now employed by various manufacturers.
Chemical vapor deposition techniques for producing high purity silica glasses were originated in at least the 1930's. U.S. Pat. No. 2,272,342 issued to J. F. Hyde teaches a vapor deposition technique for making transparent fused silica. Vapors of halides of glass forming oxides are fed through an oxy-gas burner to oxidize and form sub-micron particles of glass (generally called "soot"). This soot is deposited at low temperatures as a porous preform or at higher temperature in the form of a clear glass. Also it is disclosed that the porous glass preform can subsequently be heated to sinter by viscous flow to form a clear glass. Later U.S. Pat. No. 2,326,059 to M. E. Nordberg teaches fabricating a silica glass doped with titania, which raises the refractive index of the body along with lowering its expansion coefficient.
U.S. Pat. No. 3,737,292 issued to D. B. Keck is illustrative of one widely used technique for forming optical waveguides. The patent describes deposition on a glass target rod to form a preform for optical fiber, in which process the target glass rod is subsequently removed prior to sintering because it is considered undesirable to leave it as a part of the preform and the resultant fiber. A central hole in the core structure is collapsed during the subsequent drawing step. Although the central void is ostensibly filled in this process, there are in fact some discontinuities introduced that tend to affect light propagation characteristics as in U.S. Pat. No. 4,157,906 to Bailey. Also, to minimize stress breakage, complex composition and viscosity gradients need to be built into the preform as in U.S. Pat. Nos. 4,344,670 (Blankenship), 4,358,181 (Gulati et al), and 4,251,251 (Blankenship). Other U.S. patents that describe methods of fabricating preforms by radial vapor deposition where the target rod or mandrel is removed, commonly known as the outside process, are U.S. Pat. Nos. 3,823,995; 3,884,550; 4,135,901 and others. U.S. Pat. No. 3,775,075 issued to D. B. Keck describes deposition of cladding glass of lower refractive index on a polished glass rod which acts as the core of the fiber and is not removed. In this process, the core rod must be properly dimensioned and finished in order to provide a proper core when the preform is drawn down to fiber diameter. Even so, the surface of the core rod can have imperfections, and the discontinuity between the core rod and deposited cladding tends to introduce seeds during subsequent processing steps.
A later variant of this technique is disclosed in U.S. Pat. Nos. 4,298,366 and 4,423,925, in which the original start rod is a cylindrical glass rod of pure fused silica on which a core layer of borosilicate soot of varying refractive index is deposited. After sintering, the preform is drawn down to a smaller size and a cladding layer is then deposited for final vitrification and drawing to an optical fiber. The area of the original pure fused silica rod is, in this structure, less than 10% of the cross-sectional area of the fiber so as to have the effect of a gradient index fiber despite the presence of the constant index region at the center. There are, however, two discontinuities in this structure, at the boundaries between the original start rod and the core, and between the sintered core and outer cladding. Minute surface imperfections in either boundary region constitute sites for the growth of defects. These approaches utilizing borosilicate compositions whether they are gradient index multimode or for single mode designs, cannot be used to produce low loss fibers at 1.3 .mu.m because of infrared absorption edge characteristics of boron trioxide.
Another process category where preforms are made by deposition on the inner surface of a substrate tube (known as the inside process) are described in U.S. Pat. No. 3,711,262 and others. The inside processes also suffer from severe limitations in preform size due to restrictions imposed by the substrate tube. Furthermore substrate tube geometrical tolerances and glass quality have strong impact on fiber geometry control and fiber strength. Again, the structure must be collapsed to fill a central void, so that imperfections tend to appear, reducing quality or yield or both.
Yet another process category, using deposition axially on the end of a mandrel, is called the vapor axial deposition process. This process is described in U.S. Pat. Nos. 3,966,446; 4,017,288; 4,135,901; 4,224,046; 4,231,774 and others.
Hybrid techniques whereby a core is formed by axial deposition and a cladding layer is formed simultaneously by radial deposition are described in U.S. Pat. Nos. 3,957,474 and 4,062,665. Substantially continuous methods of forming optical fibers by vapor deposition are described in U.S. Pat. No. 4,230,472. In this patent a substantially continuous core member is longitudinally translated on which cladding glass is deposited, the porous preform being sintered and drawn into fiber continuously.
The axial process produces fiber with excellent transmission characteristics. However, preforms from this process suffer from axial variability of dimensions as well as refractive index profile. Also, with the axial deposition process it is difficult to produce fiber with the high cladding to core ratio required for single mode fiber. Furthermore, there is a need to control temperature very precisely on the deposition surface, and this limits the rate at which material can be deposited.
The outside vapor deposition process, which is really a radial vapor deposition process utilizing a removable mandrel, has produced the largest preform and has demonstrated the highest deposition rate per burner. However, the necessity of mandrel removal restricts the length of preform that can be made using this process and adds an additional process step of mandrel removal. In addition the force of mandrel removal can damage the inside surface of the preform causing seeds in resultant fibers adversely affecting optical performance by introduction of light scattering and attenuation.
A variant on this technique that suffers from the same problems is disclosed in U.S. Pat. No. 4,298,365, in which successive layers of glass soot are deposited on a removable cylindrical mandrel. A stratum of low viscosity glass soot is applied, followed by first and second glass soot coatings of successively higher viscosity but with different indices of refraction. The differing viscosities are required because of the necessity of collapsing the central aperture formed when the mandrel is removed. Although the stratum is required to be thick enough to form a continuous layer after mandrel removal, it still constitutes an irregular surface that tends to introduce defects that attenuate light or render the optical fiber non-concentric.
Processes of vapor deposition on a removable mandrel or glass target, or on the inside wall of a tube, require ultimately that the structure be collapsed to close the center hole. Such collapse is difficult to achieve without creating an irregularity in the center refractive index due to outer diffusion of dopants. Furthermore, core/cladding concentricity can suffer, especially when deposition is performed inside a tube, since it is difficult to make tubes with truly concentric walls.
The hybrid or continuous processes described may not be economically viable for fiber manufacturing, due to their complexity.
Because the economics of the preform fabrication step in producing optical fiber primarily depend on the rate and efficiency of deposition, whereas the economics of downstream process steps in the fiber manufacturing primarily depend on the preform size, it is important to provide a process for producing optical fibers which makes large preforms at high deposition rates, with tight control on fiber parameters, including maintaining the core and cladding dimensions, in order that the resultant fiber be useful in actual practice. The core and cladding should have the desired difference in refractive index but also should contain a minimum number of interfaces to minimize the entrapment of tiny air bubbles and foreign particles at the interfaces, and the creation of other irregular sites of potential defects in the optical fibers. Interface defects in a preform are typically very minute and are not in any event subject to analysis except by destructive testing in the preform. They do not, however, diminish proportionally in size when the preform is drawn down to a very thin fiber. Instead, the seeds in the interface region become large relative to the fiber diameter and define light scattering centers at the interface regions. The volumetric density of the seeds directly affects the light transmissivity of the fibers. Other seeds can also be present, in the interior of a core or cladding volume, but these arise from imperfect deposition or sintering and must be separately confronted.
Improved transmissivity is constantly being sought in the current state of the art, and to this end workers in the field have also sought to minimize the losses to interface defects by placing the interfaces in a region containing relatively low light energy in the fiber structure. The confined light waves have a Gaussian energy distribution centered about the fiber axis and dropping to a low level where the ratio of cladding thickness (t) to core radius (a) is substantial. The practice has been to use a t/a ratio of 6 or 7 so that the interface is in the region of the skirts of the Gaussian curve, largely because dissimilar surfaces provided by the known processes have created levels of defects that did not tolerate other relationships.